As the elements incorporated into a semiconductor device are increasingly integrated, the structure of device wires such as gate lines and bit lines continues to become multiple-layered. For this reason, step coverage between unit cells on a semiconductor substrate is increased. To reduce the step coverage between the unit cells, various methods of polishing a wafer have been developed. Among these methods, a chemical-mechanical polishing (CMP) method, which planarizes a polished surface (processing surface) of the wafer during fabrication, is widely used.
In a general CMP process, a polishing head of a CMP apparatus secures a wafer using a vacuum or surface tension and loads the wafer on an abrasive pad of a turntable. The polishing head imposes a controllable load on the wafer to hold it in tight contact with the abrasive pad. Thereafter, the polishing head may be rotated to rotate the wafer with respect to the abrasive pad of the turntable.
In order to increase the efficiency of the CMP process, the wafer should be polished at a high speed while maintaining uniform flatness. However, characteristics such as uniformity, flatness and polishing speed of the wafer are highly dependent on relative speed between the wafer and the abrasive pad, as well as the force or load of the polishing head urging the wafer against the abrasive pad. Particularly, the larger the force imposed on the wafer by the polishing head against the abrasive pad, the faster the polishing speed. Accordingly, in the case where an uneven load is imposed on the wafer by means of the polishing head, a portion of the wafer on which relatively large force is imposed will be polished at a faster rate than other portions of the wafer on which relatively small force is imposed.
Generally, the polishing head includes a flexible membrane which is adapted to pick up and release the wafer by vacuum. However, the vacuum between the membrane and the wafer often times leaks, such that during transfer, the wafer may be dropped or otherwise harmed.
To address these limitations, a polishing head with a modified structure has been proposed, which chucks/releases a wafer via vacuum holes formed at bosses that protrude from a chucking supporter of the head. However, such a polishing head introduces limitations that are shown in FIG. 1, which is a graph illustrating the resulting uneven surface of a wafer. In FIG. 1, reference character A indicates a wafer portion corresponding to the protruded bosses and reference character B indicates a wafer portion corresponding to a step projected from an edge of the supporter. Portions A and B are relatively over-polished as compared to other portion of the wafer, thereby compromising the uniformity of polishing surface of the wafer.
Polishing uniformity in the CMP process depends highly upon the equipment used, particularly the structure of the polishing head. For this reason, the CMP industry has eagerly developed and applied membrane-type heads of a high polishing uniformity. Further, as the wafer caliber becomes larger, there is a high demand for equipment adapted for controlling the CMP polishing characteristics at regions near the edges of the wafer.